Many industrial facilities utilize fuel cell power plants to satisfy both their daily and temporary need for electricity. The fuel cell itself consists of an anode, a cathode and an electrolyte that separates the two. Fuel reactant gas, which is typically a hydrogen rich stream, enters the anode of the fuel cell and oxidant reactant gas, which is commonly air, enters the cathode of the fuel cell. A catalyst in the anode causes the hydrogen to oxidize resulting in the creation of hydrogen ions, which pass through the electrolyte to the cathode, thereby creating an electric potential across the fuel cell.
There are various types of fuel cells, which vary according to their electrolyte. The electrolyte is the ionic conducting substance between the anode and the cathode. One type of fuel cell includes an acid electrolyte, such as phosphoric acid (H.sub.3 PO.sub.4), typically operating at a temperature range of about 375.degree. F. to about 425.degree. F. Most fuel cells, therefore, include a cooler that allows coolant fluid, such as water, to circulate through the fuel cell in a re-circulation loop, thereby controlling the temperature of the fuel cell within a predetermined range. As the coolant fluid passes through the acid electrolyte fuel cell, however, the temperature of the coolant fluid increases to its saturation temperature such that a portion of the water begins to boil and transforms to its water vapor stage. At this point, the temperature of the coolant fluid remains relatively constant as additional water transforms to steam. By way of example, a typical phosphoric acid fuel cell cooling loop generates approximately 115 psia steam at a saturation temperature of about 338.degree. F. This steam is thereafter separated from the liquid by a steam separator and utilized elsewhere within the fuel cell power plant. Namely the steam is utilized by a fuel processor or for other cogeneration processes that may require such a heat source. The fuel processor is a device that converts (i.e., reforms) a hydrocarbon fuel into a hydrogen rich stream, which is used by the anode.
Another type of electrolyte includes a membrane electrolyte, such as a solid polymer electrolyte or otherwise referred to as a proton exchange membrane (PEM). The solid polymers are commonly sulfonated fluorinated polymer membranes similar to those sold by E.I. dupont de Nemours and Company under the trademark NAFION. Fuel cells incorporating a solid polymer membrane or proton exchange membrane will hereinafter be referred to as a PEM fuel cell, which usually operate at a temperature range of about 140.degree. F. to about 200.degree. F. Such fuel cells typically operate in a pressure range of from about one atmosphere to about five atmospheres. The temperature and pressure of a PEM fuel cell are below the boiling point of water at about one atmosphere (i.e., ambient pressure), thereby preventing the coolant fluid from serving as a natural source of steam.
Although the phosphoric acid electrolyte fuel cell serves as a source of steam, its inclusion within a power plant increases the overall cost as compared to a PEM fuel cell because the power plant must include certain equipment and be constructed of materials capable of withstanding the operating environment, including the coolant pressure associated with a temperature range of about 375.degree. F. to about 425.degree. F. Furthermore, designing a system utilizing a phosphoric acid fuel cell, as compared to a PEM fuel cell, yields a lower actual power output for the power plant because the PEM fuel cell has higher performance characteristics. It is desirable, therefore, to design a fuel cell power plant by replacing the acid electrolyte fuel cell with a PEM fuel cell, thereby reducing the overall material and assembly costs associated with construction of such a power plant as well as eliminating compliance with the ASME pressure vessel codes.
The PEM fuel cell, however, does not operate at a temperature sufficient to cause the coolant fluid to boil and transform to steam, which is needed by the fuel processor to convert the hydrocarbon fuel to a hydrogen rich fuel stream. What is needed is an efficient method of converting the coolant stream of PEM fuel cell to steam.